The method of signal reception generally used for television receivers at present is the intercarrier method, i.e. a method whereby the video and audio components of the modulated radio frequency television signal applied from a receiving antenna are processed within common circuit stages, to derive video and audio signals. The intercarrier method has the disadvantage that if the video signal carrier component of the antenna signal should fall to a very low level, i.e. due to fading, the audio signal output will cease together with the video signal output. Another reception method used for television reception is the split-carrier method. This has the disadvantage that it is necessary for satisfactory reception that the difference in level between the audio and video carrier components of the received antenna signal be substantially equal to 6 dB.
The audio and video carrier components of a television signal, although closely adjacent in frequency, occupy different frequency bands and employ different methods of modulation. That is to say, amplitude modulation (AM) is used for the video carrier, and frequency modulation (FM) for the audio carrier. Frequency modulation is very susceptible to multi-path fading interference, i.e. fading which results from radio waves reaching the receiving antenna along two or more different paths, for example due to reflections from aircraft flying nearby, in the case of a stationary television receiver installation, or due to reflections from large buildings, in the case of a mobile television receiver installation. However in the case of the audio carrier modulation component, since frequency modulation is used and since the modulation level is lower than in the case of FM audio broadcast signals, the audio signal produced by the television receiver is comparatively insensitive to the effects of multi-path fading. Multi-path fading interference will therefore have a much more serious effect upon the picture quality of a television receiver than upon the sound quality. In addition, due to the fact that the video and audio carrier components of the TV signal occupy different frequency ranges, these components will be affected differently by multi-path fading. Thus, it is possible that a maximum level of audio carrier component is being received from the antenna at a moment when the level of video carrier component is close to maximum. The requirements for operation of automatic gain control (AGC) circuits are therefore basically different, for the audio and video carrier components respectively. However in prior art types of television receiver, an AGC control signal derived on the basis of the level of received video carrier component is utilized to control the gain of common input circuit stages which process both the video and audio carrier components of the received antenna signal. Such input circuit stages are generally referred to collectively as the "front end" circuit, and essentially comprise frequency conversion circuits for converting the antenna signal to a lower frequency, and intermediate frequency (IF) circuits for amplifying this frequency converted signal within a specific bandwidth.
Furthermore, if diversity reception employing two or more antennas is adopted, to overcome the effects of multi-path fading, it is not possible to obtain satisfactory performance with respect to both the video and audio components of the television signal, with a conventional type of television receiver in which a common front end circuit is used to process both the audio and video carrier components of the received antenna signal. Selection of the antenna to be currently connected to the common front end circuit, in such a case, will be made upon the basis of the level of received video carrier component. Thus, it is possible that the selected antenna, while providing a satisfactory level of video signal will actually provide an excessively low level of audio signal. It can therefore be appreciated that, as in the case of AGC control, control of antenna selection in a television receiver employing diversity reception cannot be performed in a satisfactory manner with regard to both the audio and video components of the television signal, with a prior art type of television receiver in which a common front end circuit is used for the initial stages of frequency conversion, selection and amplification of the received antenna signal.